A typical vector network analyzer (VNA) has the ability to measure device under test (DUT) impedance when the DUT impedance is close to the characteristic impedance of the VNA to a good precision and up to about a 100 gigahertz (GHz) stimulus frequency. For example, a typical characteristic impedance of a VNA is 50 ohms.
A VNA, for example, measures the reflection coefficient resulting from the DUT impedance in order to determine DUT impedance. The reflection coefficient directly varies with the DUT impedance.
When the DUT impedance is close to 50 ohms, the value of the reflection co-efficient for the DUT is very sensitive to changes in DUT impedance. This results in a low signal-to-noise ratio (SNR) when using the value of the reflection co-efficient to determine the DUT impedance. Thus it is possible to determine a value for DUT impedance to a high resolution.
When the DUT impedance is much less than 50 ohms (e.g. less than 5 ohms), the value of the reflection co-efficient for the DUT is not very sensitive to changes in DUT impedance. This results in a high signal-to-noise ratio (SNR) when using the value of the reflection co-efficient to determine the DUT impedance. In this case, it is possible to determine a value for DUT impedance only to a lower level of resolution.
Likewise, when the DUT impedance is much greater than 50 ohms (e.g. greater than 400 ohms), the value of the reflection co-efficient for the DUT is not very sensitive to changes in DUT impedance. This results in a high signal-to-noise ratio (SNR) when using the value of the reflection co-efficient to determine the DUT impedance. In this case, it is possible to determine a value for DUT impedance only to a lower level of resolution.